Cleanup cassandra protocol negotiation

[rukai]

Improves error reporting and makes the cassandra protocol implementation easier to understand.
Downgrades the warnings caused by cassandra protocol negotiation to info! level.
It certainly makes the integration tests quieter, but maybe the slight performance loss of protocol negotiation is worth a warning?

Theres some magic trait type stuff going on here:
In these lines:

pub trait CodecReadHalf: Decoder<Item = Messages, Error = CodecReadError> + Clone + Send {}
impl<T: Decoder<Item = Messages, Error = CodecReadError> + Clone + Send> CodecReadHalf for T {}

We can set any type to be the Error as long as it implements From<std::io::Error>.
Internally tokio then uses that implementation to generate errors in our type when it hits an io::Error while receiving from the TcpStream.
So while we never create a CodecReadError::Io directly, tokio will return those sometimes when we call FrameRead::next.

[conorbros]

Let's merge this, I assume Ben just forgot to hit the resolve button.